Q.Consider the following electronic arrangements for the $d^5$ configuration:

Hund's rule of maximum multiplicity fills the one gap the Aufbau principle leaves open: it decides how electrons are distributed among a set of several DEGENERATE orbitals of equal energy (such as the three p, five d, or seven f orbitals of one subshell), which the Aufbau principle - concerned only with the ORDER subshells fill in - does not address. The rule states that electron pairing within such a degenerate set does not begin until every orbital in the set already holds one electron each, all with parallel spin; only once every orbital has one electron does a second (opposite-spin) electron start pairing up in any of them. Carbon ($1s^2\,2s^2\,2p^2$) illustrates this: its two 2p electrons occupy two SEPARATE p orbitals ($2p_x^1\,2p_y^1$) rather than pairing together in one ($2p_x^2$), because electrons kept in separate orbitals are on average farthe …